1. Field of the Invention
The present invention relates to a multi-phase PM-type stepping motor suitable for office automation equipments such as printers, high-speed facsimile equipments, plain paper copiers (PPCs), and so on.
2. Description of the Related Art
A single-phase PM-type stepping motor which is a first example of prior art will be described with reference to FIGS. 34 and 35.
FIG. 34 is a vertically-sectioned side view of a single-phase PM-type stepping motor 30 which is a first example of prior art. In FIG. 34, the reference numeral 31 represents a cylindrical rotor in which N (north) and S (south) poles are formed alternately in the circumferential direction in the outer circumferential surface thereof; 32, a rotating shaft of the rotor 31; and 33, a ring-like stator disposed so that the inner circumferential surface thereof is opposite to the outer circumferential surface of the rotor 31 through a gap.
The reference numeral 34 represents a stator coil constituting the stator 33; 35, a stator yoke; 36a and 36b, motor mounting plates fixed to the stator yoke 35; and 37a and 37b, bearings provided on the mounting plates 36a and 36b respectively for rotatably supporting the rotating shaft 32.
Next, the stator 33 will be described in detail with reference to FIG. 35. FIG. 35 is a vertically-sectioned side view illustrating the stator 33 in an exploded state.
The stator yoke 35 is constituted by one and the other ring-like yoke elements 39a and 39b provided with comb-like pole teeth 38a and 38b extending in the axial direction along the inner circumferential surface of the stator coil 34. The pole teeth 38a and 38b are disposed so as to be adjacent to each other while shifted from each other in the circumferential direction.
The stator coil 34 is constituted by a ring-like bobbin 40 surrounded by the yoke elements 39a and 39b and the pole teeth 38a and 38b, and a ring-like coil 41 wound on this bobbin 40.
Next, description will be made about a two-phase PM-type stepping motor, which is a second example of prior art, with reference to FIG. 36.
FIG. 36 is a vertically-sectioned side view of a two-phase PM-type stepping motor 50 which is a second example of prior art. In this stepping motor 50, a stator 53 is constituted by first and second stator portions 53a and 53b ganged together in the axial direction.
In the single-phase PM-type stepping motor described above as the first example of prior art, however, the stator is constituted by a single stator portion. Accordingly, in order to determine the direction of rotation of the motor, the pole teeth of the stator, and the magnetic permeance or phase between magnetic poles of the rotor are necessary to be shifted properly. Alternatively, it is necessary to use any mechanical means to determined the direction of rotation of the motor. Such a motor is not therefore suitable for an equipment which requires high-speed rotation or high torque.
On the other hand, according to the two-phase PM-type stepping motor as the second example of prior art which has been developed to solve the problems in the first example of prior art, the torque and rotation speed can be improved on a large scale, but it has problems in the points as follows.
(1) The number of lead wires for the coil is large to be four, and it is necessary to use at least 8 transistors to constitute a driving circuit.
(2) When the step angle is set to be fine, it is necessary to form a number of pole teeth, thereby causing a problem in construction work.
(3) It is difficult to obtain high torque with a fine step angle.
(4) In a three-phase PM-type stepping motor disclosed in JP-A-1-259748, the phase angle of current is 60.degree., and there is a problem that it is impossible to perform drive of 120.degree. current-conduction.
(5) In addition, when an multi-phase stepping motor according to the above prior art is provided as a 5-phase stepping motor by setting the number of stator cores n to be n=5, it is necessary to provide at least 10 lead-out terminals for the stator coil, and it is required to use at least 20 transistors to constitute a driving circuit.
Therefore, there arises a problem that the driving circuit is so complicated that the cost increases on a large scale.
It is therefore an object of the present invention to solve the foregoing problems, and to provide a multi-phase stepping motor in which the number of lead wires is reduced to simplify a driving circuit, while high torque can be outputted with a very fine step angle, and which can be manufactured at a low price.